Hydrocarbon conversion processes, for example steam cracking, visbreaking, catalytic cracking and coking, are carried out at high temperatures to produce a wide variety of olefinic compounds such as ethylene, propylene, n-butene-1, n-butene-2 compounds, isobutene or pentenes, and diolefinic compounds such as 1,2-propadiene, 1,3-butadiene and other compounds with boiling points in the "gasoline" cut range and which can be olefinic or diolefinic. Such processes inevitably lead, however, to the formation of highly unsaturated compounds such as diolefins (for example 1,2-propadiene), also alkynes (for example acetylene, propyne, 1-butyne, etc.). Such compounds have to be eliminated to allow the different cuts from these processes to be used in the chemical industry or for polymerisation processes. As an example, the C.sub.4 cut from steam cracking contains a high proportion of 1,3-butadiene, butene-1, butene-2 compounds and isobutene.
Conventionally, butadiene is separated from the olefinic cut, for example by extractive distillation in the presence of dimethylformamide or N-methylpyrrolidone. The olefinic cut thus obtained contains isobutane, isobutene, butene-1, butene-2 compounds, n-butane and 1,3-butadiene, the latter being in an amount which can be between 0.1% to 2% by weight.